1. Field of the Invention
The present invention relates to selective leaching to obtain a residual material with improved properties and, more particularly, to selective leaching of the different components of blast furnace slag using a controlled process and array of chemicals to obtain a residual silica alumina framework which has considerable surface area and catalytic activity.
2. Description of the Background
The concept of conversion of an inert slag to a catalyst support having surface area and porosity is known in the art. For example, U.S. Pat. No. 4,545,797 to Elattar teaches a process for the manufacture of porous slag from inert slag. Elattar discloses the process for the manufacture of slag having high surface area comprising the following steps (i) calcining the slag (ii) grinding the calcined slag to a size 0.1-0.3 mm (iii) leaching the ground material with aqueous acid having a pK.sub.a of 1-5 or base having a pK.sub.b of about 2-6 or a combination thereof. The Elattar process is a one step non-selective leaching operation at an unspecified high temperature. This indiscriminate leaching at an indiscriminate temperature leaches out the desired alumina required for acid catalysis. Moreover, Elattar's leaching agents are NaOH or HF, both of which dissolve silica (see column 3, line 21). As a result, the end product of Elattar's process yields an unspecified framework with surface areas of 13.5 m.sup.2 /g and having "macro-porosity" (therein defined as an average surface area of at least 200 m.sup.2 /g). The resulting slag, representing the synthesis of a high surface area material from an essentially zero porosity and non-existent surface area slag, can be used as a catalyst support, or in building materials, or in the ceramic industry.
There remains a need for a more controlled and selective process for slowly leaching undesired metal oxides in a controlled fashion. It is desirable to produce an end product having micro-porosity with surface areas an order of magnitude greater (between 500 and 200 m.sup.2 /g). In general, the smaller the pores, the greater the surface area, adsorption, and chemical reactivity/selectivity.
U.S. Pat. No. 4,803,188 to Aulich et al. discloses the synthesis of a heterogeneous catalyst based on silicon dioxide and incorporating other metal oxides insoluble in mineral acid. Mined silica is used as a starting material and to it glass-forming additives of aluminum oxide, carbonates of alkali and alkaline earth metals, and desired catalytically active metal oxides (e.g. oxides of iron or zinc) are added and the mixture is heated to form a homogenized melt which is converted to fiber form and the resulting fiber is treated with hot acid to leach out the glass forming additives and other impurities. The resulting porous silicate material has the desired catalytically active metals incorporated into its framework. The fusion of metal oxides having a soluble component and an insoluble component to synthesize a catalyst is also taught by Aulich et al. It is well known that alumina can be leached from amorphous SiO.sub.2 /Al.sub.2 O.sub.3 (e.g., metakaolin clay) with acid yielding a silica with meso-porosity. Therefore, the Aulich method cannot produce a silica/alumina with micro-porosity. Aulich also has the disadvantage in that any occluded alkali metals would be unleachable and destructive to any surface area in high temperature applications.
In summary, neither of the processes of Elattar nor Aulich are capable of producing a silica/alumina micro-porous material, and neither patent teaches or suggests specific products that can be obtained by the selective and controlled removal of the calcium and/or magnesium oxides from blast furnace slag, nor the resulting hierarchy of dissolution from the controlled leaching, nor the improved properties such as greatly increased surface area that can be attained by giving one the choice and flexibility from hierarchical leaching.